...However, despite significant progress recently, several key research challenges still need to be overcome to enable the mass production of silicon photonic technology.

These challenges include developing (i) a low cost method of comprehensively testing at the wafer scale; (ii) a passive alignment coupling technique from fiber to optical chip; (iii) a means of scaling the functionality of the photonic circuit; (iv) very low power, high data rate modulators; and (v) low-cost integrated lasers within silicon photonic chips.

I'd switch between (i) and (ii) in importance, I don't understand what they mean by (iii), (iv) is always true (better and cheaper) but i'm not sure the current data rate and power consumption are the obstacles for market penetration. (v) is interesting. It sure is important for WDM, but I wonder if there is no model where there are external light sources (like the chip's electric power supply).